An Ocean of Air (11 page)

There arose from a piece of heart of oak, 216 times its bulk of air. Now 216 cubick inches of air, compressed into the space of one cubick inch, would, if it continued there in an elastick state, press against ... the six sides of the cube with a force equal to 19860 pounds, a force sufficient to rend the Oak with a vast explosion.

Since Hales was no fool, and he had noticed that oak trees do not generally explode without warning, he decided that the air he had released must somehow have previously been fixed in place. Hales imagined that his "fixed air" was made of particles that repelled each other mightily. In some circumstances, he believed these particles could become bound inside his solid objects and in others, liberated again.

However, all Hales cared about was how air became fixed, and how it subsequently recovered its bounce. He had no idea what was truly happening when gas flooded so unexpectedly out of a solid, nor did he realize that individual "airs" might have different properties.

***

Steady, thoughtful Joseph Black was much better placed than the rambunctious Hales to figure this out. Inspired by Hales's work, he wondered whether his
magnesia alba
had been transformed by losing some quantity of fixed air. That, at least, would explain why it lost so much weight. Moreover, rather than assuming that every air was the same, just with more or less bounce, Black suspected that Hales's fixed air might have properties of its own, ones that could be quite different from those of ordinary, common air. Perhaps it even had enough individual characteristics to explain why the
magnesia alba
had lost its causticity and turned so mild after it vanished.

Black didn't manage to catch any gas in the act of escaping from
magnesia alba,
but had more success with one of its caustic cousins: marble. He heated a cubic inch of the stuff, and—sure enough—produced a huge amount of fixed air, enough to fill a vessel holding six gallons.

Now that he had some samples of fixed air to work with, Black determined to find out whether it was truly different in character from common air. The experiment he designed to test this was rather complicated, but also ingenious. Black knew that limewater (which is just lime, or calcium, dissolved in water) had an affinity for fixed air. He decided that the lime in limewater must be soaking up the air, the exact opposite reaction to the way that
magnesia alba
and marble had released it. He also knew that water always has a certain amount of common air dissolved in it. That is why fish can breathe underwater, and why tiny bubbles form long before a pot of water comes close to boiling.

So he wondered what happened to the common air dissolved in limewater. If common air were just the same stuff as fixed air, any common air in limewater would be sucked up by the lime, leaving none behind in the water. Black realized that all he had to do was check how much common air was dissolved in equal quantities of ordinary water and limewater. If equal amounts came from both, the air the lime sucked up must be fundamentally different. And that would mean that his new air really was special.

To put his idea into practice, Black needed an air pump. But the only one available in Edinburgh was frustratingly out of action, and its slow, surly technician was impervious to Black's pleasant requests that he speed up his attempts to fix it. Exasperated, Black wrote to his former tutor in Glasgow, begging him to use the air pump there, and explaining with great precision exactly how the limewater should be made and treated. His tutor quickly arranged for the experiment to go ahead. Word came back. Four ounces each of limewater and ordinary water had been placed under the receiver of the new Glasgow air pump. As the pump sucked, air bubbled up out of each of the two vials. Each released almost exactly the same amount.

Black was delighted. "From this it is evident," he wrote in his thesis, "that the air which quicklime attracts, is of a different kind from that which is mixed with water.... Quicklime does not attract air when in its most ordinary form, but is capable of being joined to one particular species only, which is dispersed throughout the atmosphere." In honor of Hales, Black decided to call this extraordinary new species fixed air. We know it now as carbon dioxide.

In the history of science, this apparently innocent moment was in fact extraordinarily profound. For this was the first time that anyone had shown there was more than one kind of gas. Because of this discovery, Black would be known as the father of modern chemistry. Lavoisier, Priestley, and their contemporaries all regarded themselves as Black's disciples. Lavoisier, usually reticent about offering credit to others, even wrote to Black saying how much he admired his work.

But more important for our story is the nature of the gas he had found. Ever curious, Black decided to abandon his work on bladder stones for a while and find out how his new fixed air behaved. He remembered the old experiment that he had described back in January to his tutor. Sure enough, adding acid to chalk produced the same fixed air that had flooded out of
marble. Black also found that he could make it by simply burning charcoal in ordinary air. And as before, though the fixed air smelled "not disagreeable," it snuffed out candles, and animals could not breathe it and live.

Black also noticed that fixed air is a product of distillation and that it appears in our breath when we exhale. He was, however, baffled by what it could be doing in our bodies in the first place. "It is not to be doubted," he wrote, "indeed that this air, extensively united with every part of our body, serves many great uses, nor is it to be supposed that its absence could be borne without inconveniences: but we do not seem to know what its use is, or what are the inconveniences that would result from its absence."

The "inconveniences," it turns out, are that without it we and most other living things on Earth would starve to death.

Black never knew the vital role that his discovery, carbon dioxide, plays in our lives, but those who came after him quickly began to recognize its importance. In Lavoisier's experiments with respiration, he realized that the more oxygen a person or animal consumed by breathing, the more "fixed air" they produced. He deduced that we burn our carbon-based food in much the same way that a candle burns its carbon-based wax, and for the same reason: to release energy. And burning carbon-based substances in oxygen produces—what else but carbon dioxide.

Priestley, meanwhile, spotted that the interplay between fixed air and oxygen was somehow related to plants. He knew that a mouse in an enclosed chamber would eventually be unable to breathe, but he discovered that placing a plant in the same chamber kept the air from getting noxious indefinitely. The plant and mouse seemed to work in contented cooperation to keep the air fresh.

This is not merely a curiosity. Subsequent scientists have discovered that it's the fundamental basis for life as we know it on Earth. For the existence of carbon dioxide, and its relationship with oxygen, is the foundation for a pact between plants and animals the world over.

We animals take in oxygen to burn our food and throw out carbon dioxide as a waste product. Plants work the other way around. They take in carbon dioxide to
make
food and produce oxygen as
their
waste product. (Plants also need to breathe, to release energy from the food they make. They use up about a quarter of the oxygen they produce, but the rest they leave for us.) So we have a deal that keeps us all alive—plants soak up our leavings and we soak up theirs. Air is the living, breathing medium for this eternal interchange.

The plant's side of this bargain is the basis for all food production on Earth. The first hint that this might be so had come in the mid-seventeenth century, when a Dutch alchemist named Jan Baptista van Helmont performed a curious experiment. He had begun to wonder what plants are made of, or more particularly where the stuff that makes a plant comes from. So he took a large pot, and in it put two hundred pounds of earth that he had carefully dried in a furnace. In this pot he planted a young willow sapling, weighing five pounds. And over the top of the pot, so that no extra dust could enter from the air, he fitted a metal plate full of holes around the sapling's trunk. Van Helmont was a persistent fellow. He pursued his experiment for a full five years, watering, watching, and waiting. In the end, he had a towering willow tree that weighed "169 pounds and about three ounces."

So where had the tree come from? The first thing to test was the earth in the pot. Van Helmont removed the earth, dried it, and weighed it. It had lost a mere two ounces.

This might not seem so surprising. After all, anyone who has ever owned a house plant knows that it will grow happily without your adding new soil to the pot. But in that case, what had made the willow tree's branches, trunk, and leaves?

Van Helmont guessed wrongly. The only thing he had added to the pot was water, so he blithely declared that water had to be the source. (He wasn't brilliantly logical in his deductions in other matters, either. Among other odd beliefs, he was convinced that living things could arise spontaneously out of the strangest ingredients. He even published a recipe for making mice out of dirty underwear and wheat: "For if you press a piece of underwear soiled with sweat together with some wheat in an open mouth jar, after about 21 days the odor changes and the ferment coming out of the underwear and penetrating through the husks of the wheat, changes the wheat into mice.")

The problem in this case was that he hadn't even noticed that the tree was surrounded by something else that was a superb source of raw material for making plants: thin air. The source of every ounce of the solid roots, trunk, branches, and leaves of Van Helmont's willow tree was the carbon dioxide in the air around it. When plants soak up carbon dioxide, they take air and turn it into the food that eventually finds its way into our stomachs.

Plants do this in a complex series of internal reactions, but the overall result is a simple one. They use the sun's energy to break apart carbon dioxide and turn it into the carbon-based molecules that make up our food. The scale of this activity is staggering. Every year, green plants convert carbon dioxide into 100,000 million tons of plant material. To do this, plants use up 300 trillion calories of energy from the sun, which is thirty times the energy consumption of all the machines on Earth. Even the animals we eat gain their protein and fat from plant food. Carbon dioxide in our atmosphere is the fundamental foodstuff for every plant, animal, and human on the planet.

Trees and plants take their nutrients from our ocean of air in the same way that waving fronds of seaweed do from seawater. And when we breathe, we simply recombine the food they made with the oxygen they produced to start the process all over again. The balance isn't perfect, and that turns out to be a good thing. The only reason we have oxygen to breathe in the atmosphere today is that plants keep hold of a certain percentage of the stuff they make and prevent us animals from eating it, breathing it, and turning it back into carbon dioxide. The fraction is small, just 0.01 percent of the stuff that plants make, but that also means the same percentage of the oxygen they make also remains free to float up into the sky. Over billions of years, this has built up into the atmosphere we need to live.

Some researchers even see the pact between plants and animals as being more like a battle. At certain times in the past, plants have had the upper hand. For instance, a little over 400 million years ago, plants discovered how to make lignin, the hard stuff that turns into the woody parts of
trees. Nothing in the animal kingdom knew how to digest this strange new material, so it remained untouched and unrespired—and a little less carbon dioxide made it into the atmosphere.

Then came the two champions of the animal kingdom: termites and dinosaurs (the vegetarian sort). Both learned how to digest lignin, and carbon dioxide levels rose again. Until, that is, the extinction of the dinosaurs, when plants learned how to make vast grasslands and the balance swung again.

This mattered for much more than plant pride. It turns out that interfering with the amount of carbon dioxide in the atmosphere can have serious consequences. As well as providing our food, carbon dioxide plays another role, which is every bit as crucial in shaping our planet for life.

***

The man who discovered this was John Tyndall, an exuberant Irish physicist who was a professor at London's ultrafashionable Royal Institution in the mid-nineteenth century.

The Royal Institution was the perfect place for someone like Tyndall; he could perform his research in the basement laboratories and then talk about science in the famous lecture theater aboveground. Science had become one of the hottest entertainments in town. Lectures at the institution were so popular that, to cope with the crush of carriages, Albemarle Street became Britain's first one-way street. And it wasn't only scientists who were crowding onto the Royal Institution's uncomfortable wooden benches. There were poets and politicians, intellectuals and aristocrats, in fact most of London's
beau monde.

Tyndall loved lecturing. Perhaps because he had come to research late, beginning his higher education only in his late twenties, he couldn't wait to pass his findings on. He was less concerned about education than about sharing his own wonder. He choreographed his lectures as for a Broadway show and worried endlessly about how to ensure their success. One day when he was preparing a lecture, Tyndall knocked an instrument off the table but managed to vault over and catch it before it reached the ground.
He was so delighted with the effect that he practiced it for hours. When he "accidentally" repeated the feat that evening, he brought the house down.

These efforts paid off. When word went out that Tyndall was lecturing, the house was always packed. And not just at the Royal Institution. Tyndall's lectures to illiterate working men at the Royal School of Mines attracted audiences of six hundred or more. One contemporary commentator wrote: "Professor Tyndall has never for an instant looked upon the masses as entitled to only second rate knowledge. They have had it of the highest and purest which it was in his means to supply." And during a lecture tour in America, the
New York Daily Tribune
said of him: